After demonstrations of SBM at the Chicago Consumer Electronics Show and at
Sony’s Tokyo recording studios summer 1992, I and everyone else who heard the
comparisons agreed that SBM was a significant improvement over conventional
transfer technology. It seems, however, that there may be more to the SBM story
than first revealed—both in SBM’s technology and marketing.

At the most recent Audio Engineering Society convention in San Francisco (full
re port to follow next month), a Sony engineer presented a technical paper
on SBM. The presentation ignited a firestorm of controversy that continued
in the hallway for 45 minutes after the paper’s conclusion. Many respected
audio scientists questioned both the technique and how it has been represented
to the audio press.

On a technical level, it appears that SBM isn’t as sophisticated as was first
suggested. The promises of having additional information below the 16-bit level
(not accounting for dither) and a smaller quantization step size aren’t quite
accurate. Instead, SBM is merely a noise-shaping technique that pushes quantization
noise away from the midband (where the ear is most sensitive) to the upper
treble (where the ear is less sensitive). Period. Al though this is a valid
scheme and an improvement over no noise-shaping—pushing the noise down below
what would be the equivalent of an 18-bit system, where the ear is most sensitive,
results in improved resolution —a similar technique has been an integral part
of the Harmonia Mundi Acoustica encoder for two years.

Much of the criticism at the AES focused on SBM’s noise-shaping curve—a curve
that reportedly mimics the human ear’s varying sensitivity to different frequencies.
Professor Stanley Lipshitz—whose former student, Robert Heddle, now works for
Sony and gave the presentation—sat next to me and offered his comments throughout
the paper. Of Sony’s curve, he said, “I don’t know what creatures that curve
was designed for, but it certainly wasn’t for human beings!” Dr. Lipshitz had
prepared a graph comparing his own noise-shaping curve (developed over two
years ago) with the SBM curve (fig.1). The Lipshitz curve shows a much lower
noise floor between 2kHz and 6kHz, a region where the ear is most sensitive.
According to K. Hazaina, a Sony engineer involved in SBM development, Sony
considered the Lipshitz curve (and others) but chose their own curve after
extensive listening tests. The Sony curve looks less good on paper, but reportedly
sounds closer to the 20-bit digital source than the other curves evaluated.

Fig. 1: Comparison of Lipshitz and Heddle (Sony) noise-shaping curves. Note
that both trade off a large rise in audio-band noise above 14kHz for increased
resolution below. The Lipshitz curve offers the greater increase in HF noise,
but more closely mimics the threshold sensitivity of the human ear and gives
significantly greater resolution in the low treble where the ear is most sensitive.
(Linear frequency scale, 10dB/vertical div.)

Dr. Lipshitz also objected to the fact that SBM has garnered so much attention
when the idea isn’t as revolutionary as press reports indicated. Lipshitz published
work in this area over two years ago. His argument that the SBM curve is not
optimum may be a case of differing opinions among researchers. However, his
feeling that SBM has received undue media attention and hype is justified,
in my view, considering that the technique has been known for some time.

Similarly, several other researchers approached me to express their dismay
that SBM has gotten so much hype Further, they objected to the Sony-supplied
plots printed in my first reports One graph shows a 1kHz sinewave reportedly
with and without SBM. The SBM-processed waveform appears to have a much smaller
quantization step size— an impossible feat when the signal ends up as 16-bit.
It was publicly charged at the paper presentation that this constituted deliberate
deception: the SBM waveform had been low—pass—filtered for the graph, while
the non-SBM waveform had not, it was alleged Moreover, the noise-shaping curve
supplied by Sony showed only the noise spectrum up to 10kHz, hiding SBM’s huge
increase in noise level between 10kHz and 20kHz (see fig.1). Sony spokesperson
Marc Finer went on the record to state that both 1kHz wave forms published
in our report were subjected to identical low-pass filtering.

Robert Adams of Analog Devices suggested that SBM-processed CDs will sound
worse on CD players with poor low-level linearity. He argued that, in a fade
to silence, the silence is no longer a static digital code with SBM. Instead,
many LSBs will be toggled, continually crossing the zero transition where linearity
error is greatest, particularly if the DAC’s MSB trimmer is misadjusted or
nonexistent. This condition will add high levels of white noise to the signal.
It should be noted, however, that high-end CD player and D/A processors I’ve
measured (including both 1-bit and RI2R ladder DACs) have shown excellent low-level
linearity—usually less than <1dB error at —90dBFS and often less than <0.5dB
at —100dBFS. Although Mr. Adams’s concerns are theoretically valid, it seems
a backward step not to use SBM because some users with poor-quality converters
may hear more noise, thus denying the sonic improvements of SBM to more critical
listeners who own even moderately good converters.

[While at the dbx corporation, Bob Adams designed the first and, to date the
only 128x-oversampling, 20-bit, analog/dig ital converter. UltraAnalog bought
the rights to the design and supplies the module for high-end A/D conversion
applications (the Manley and Wadia ADCs, for example). It has remained to this
day the only 20-bit A.DC extant. Bob Adams’s original engineering prototype
is the heart of the Chesky/Bob Katz converter heard on all Chesky releases.
]

Other objections to SBM were raised at the presentation. Some criticism, however,
was clearly motivated by commercial interests: one vociferous critic has a
competing CD mastering system of lower quality on the market.

Another facet of the story reported to me was that SBM has been in use for
nearly two years on Sony Classical productions— without even a mention on the
liner notes or to the press. Indeed, Richard Schneider’s “Industry Update”
in last November’s Stereophile (p.4 suggested that SBM has been in use since
January 1992—and used on all titles in Sony Classical’s Leonard Bernstein

Royal Edition series, Kathleen Battle’s and Wynton Marsalis’s Baroque Duets,
and Emmanuel Ax’s Brahms Variations disc. It has been speculated that SBM was
conceived as an in-house technique for improving Sony’s digital transfers,
but was elevated to “break through” status and vigorously promoted when Sony
realized that they had to differentiate the sound qualities of CD and MiniDisc
for commercial reasons. It could be coincidence but SBM and MD have been promoted
concurrently. According to Finer, several test CDs were made using SBM as long
as two years ago, but were not commercially re leased in this country. He also
investigated the Kathleen Battle and Emmanuel Ax master-tape genealogy and
confirmed that SBM was not used on those titles.

Finally, it should be noted that, despite the controversy, and whatever Sony’s
motivation, SBM does indeed improve CD sound quality. Moreover, it is the first
public acknowledgement by one of the CD’s inventors that conventional 16-bit
digital audio has room for improvement. How much SBM improves CD sound quality
will be more apparent after we have firsthand experience of the process; Sony
has invited us to transfer the analog master tape of Stereophile’s most recent
recording project to digital using SBM. It will thus be possible to hear the
original analog master, the full 20-bit dig ital transfer, and the SBM-processed
16-bit. We may even include samples on our next Test CD so that you may judge
SBM for yourself After all, the proof is in the listening.